1. Field of the Invention
The present invention relates to a pigment ink having excellent dispersibility for an aqueous or oil medium and excellent dispersion stability with time, which can be used as either an water-based or oil-based pigment ink, and a process for production of the same.
2. Related Background Art
Printing inks and paints include dye based inks with dyes dissolved in solvents as colorants, and pigment based inks with solid pigments dispersed in liquid as colorants. The pigment based ink generally has an advantage that the ink is not fading, can retain a fast color, and is excellent in brightness, but also has a disadvantage that the ink tends to coagulate in long-term storage and high-temperature storage, thus rising a problem in terms of storage stability.
Hitherto, there have been many technical commentaries for production of inks from pigments, namely processes for production of inks, and dispersion of pigment is defined as a process including three steps of wetting-dispersion-stabilization. (General guidebooks include “Latest Dispersion Technology” by Gijutu Joho Kyokai Co., Ltd., issued in Jan. 16, 1993.) A various kinds of resin solution grade water-based pigment inks each using a water soluble resin as a binder and dispersant for stably dispersing the pigment in water have been proposed (Japanese Patent Application Laid-Open Nos. 58-45272, 62-95366, 62-254833, etc.). In addition, in Japanese Patent Application Laid-Open No. 10-140065, an anionic micro-capsulated pigment with a pigment covered with an organic polymer containing an anionic group is proposed. In addition, in U.S. Pat. No. 5,085,698 is disclosed a method in which an AB or BAB block copolymer having a hydrophilic segment and a segments bound to the pigment is used to provide storage stability.
An oil-based pigment ink using as a dispersant a polymer mutually soluble with a solvent to be used for stably dispersing a pigment in the solvent has been proposed (Japanese Patent Application Laid-Open No. 2000-290573).
Some of pigment inks containing conventional micro-capsulated pigments have a disadvantage that transparency, chromophoric properties, color rendering properties and the like may be degraded because dispersion of the micro-capsulated pigment is unstable and thus coagulation tends to occur, and because of large particle sizes. In addition, if the concentration of resin in the capsule is increased (the concentration of pigment is decreased), selectivity of the material for use in the ink is lowered, flexibility is compromised, and the recording liquid becomes lacking in density. In addition, if the concentration of pigment is excessively increased, it becomes difficult to produce a fine micro-capsulated pigment using resin alone, and therefore enormous manpower, equipment, energy, etc. are required, or a large amount of surfactant must be used in combination, and thus an ink record image having a satisfactory water resisting property can not always be obtained.
For the problems to be solved by the invention, the invention has as its object provision of a micro-capsulated pigment containing pigment ink being excellent in density, fineness, transparency, coloring and color-rendering properties and the like due to reduced particle size, and having excellent dispersibility and dispersion stability with time.
Another object of the present invention is to provide a micro-capsulated pigment containing water-based ink or oil-based ink excellent in freedom in selectivity of resin for inks, various kinds of additives, solvents or the like.
Still another object of the present invention is to provide a micro-capsulated pigment containing water-based ink or oil-based ink capable of achieving reduction of labor in the step of finely dispersing a micro-capsulated pigment in a dispersion medium in an ink composition, saving on enormous manpower, equipment, energy and the like, and reducing costs for producing the ink, and a process for production the same.